Ammunition, Demystified: The (non) Bubba's Guide to How Ammo Really Works

By Jeff Siewert

Ammunition, Demystified is Hatcher’s Notebook for the twenty-first century. It’s a “soft” technical document intended for small caliber ammunition design and production engineers, as well as more advanced shooters and reloaders who want a more in-depth understanding of how guns and ammunition work and interact with one another. There is very little that is truly simple about small arms ammunition due to the enormous stresses to which the bullet and cartridge case structures are subjected during the firing process and the nonlinear material properties of these ammunition components. Ammunition Demystified untangles these interactions and will help put you on the path of increased understanding of a simple yet complex topic. This book encapsulates the technical knowledge acquired on bullets, cartridge cases, barrels, and ammunition testing over a career spanning four decades. It is a useful reference for shooters, reloaders, and “ammo guys” to assist them when they troubleshoot the mechanical and dispersion performance of their firearms and ammunition.

Ammunition Demystified topics covered are

• cartridge cases and case-chamber interaction

• propellants and interior ballistics

• projectile design, aerodynamics, and stability

• trajectory, dispersion, accuracy

• barrel: manufacture, fatigue life

• ammunition: pressure and velocity testing, and load development

www.bulletology.com

• Language: English
• Publisher: Page Publishing, Inc.
• Release date: Apr 25, 2022
• ISBN: 9781662433337

Book preview

Ammunition Components

Primers

Types of Primers

Primer Sensitivity Tests

Primer Interface with the Case

Cartridge Case

Effect of Case Body Taper on Gun Function Interface Factors

Case Failure Modes

Case Material Options

Brass

Material Properties

Corrosion Protection

Steel

Material Properties

Corrosion Protection

Aluminum

Material Properties

Corrosion Protection

Plastic

Case Geometry Options

Bottleneck Cases

Straight-Walled Cases

Shotshell Cases

Case-Chamber Interaction

General Description of the Interior Ballistic Event

Gun-Cartridge Interface Parameters

Case Geometry

Peak Pressure

Material Properties

Head Space Control and Case Initial Position

Axial Gap

Radial Gap

Lock Stiffness

Coefficient of Friction

Bullet Pull / Case Crimp

Thermal Event

Residual Bolt Load at Unlock

Extraction Load

Fluted Chambers

Propellants

Overview

Muzzle Velocity Limits

Propellant Chemistry

Black Powder

Black Powder Substitutes

Smokeless Powders

Single Base

Double Base

Triple Base

Smokeless Powder Propellant Geometry

Propellant Effect on Dispersion

Group Size vs. Base Pressure at Muzzle Exit

Group Size vs. Muzzle Velocity Variation

Propellant Loading Density

Less than 100%

100% and Up

Interior Ballistics

Propellant Combustion and Selection of an Optimum Powder

In-bore Pressure Gradient

Resistance Pressure

Heat Loss

Jacket Wear

Handgun Ammunition Optimization

Projectile

Projectile Design

Solid Projectiles

Composite Projectiles

Effect of Manufacturing Flaws on Dispersion

Full-Caliber Bullets

Saboted (Subcaliber) Projectiles

Trajectory

General Description of Projectile Flight Motion

Projectile Aerodynamic Coefficients

Effect of Projectile Shape on Aerodynamics

Rifling Twist

General Discussion about Bullet Stability

Gyroscopic Stability

Dynamic Stability

Effect of Barrel Exit Twist and Land Geometry on Dispersion

Initialization Jump

The Myth of Mach 1 Turbulence

Notes on .22 Rimfire Bullets: Dispersion and Flight Behavior

Notes on Air Rifle Pellets

Ammunition/Weapon Interactions

Engraving / Action Time

Cartridge / Chamber and Bolt Face

Forcing Cone / Free Run (Why Does Seating Depth Matter?)

Projectile Shank / Ogive Shape / Rifling

Exterior Configuration

Barrel Land Width/Geometry

Base Pressure at Muzzle Exit

Gun Dynamics

Action Time / Bore Straightness Interaction

Projectile / Gun Tube Interaction

Error Budgets and Dispersion

Error Budget Definition

Dispersion (Not Accuracy, Dammit!)

Correlation of Initial Yaw and Resulting Deceleration with Observed Dispersion

Notes on Dispersion Troubleshooting

Effect of Scope Cant on Bullet Impact Point

Gun Barrel Considerations

Rifling Form

Number of Lands and Grooves

Groove-to-Land Width Ratio

Twist vs. Travel

Barrel Materials

Interior Protective Coatings

Barrel Boundary Conditions / Bore Straightness

Barrel Condemnation Criteria

Dispersion

Forcing Cone Erosion

Critical Crack Depth

Velocity Loss

Statistics

Dispersion and Dispersion Variability

Mean Point of Impact and MPI Variability

Load Development Procedure

Velocity Measurement

Pressure Measurement

Action Time Measurement

Conclusions

Appendix

Density

Strain

Modulus of Elasticity

Yield Strength

Tangent Modulus

Ultimate Strength

Ductility

Strain Rate Material Dependencies

Fracture Toughness

Glossary

References

Index

List of Figures

Figure 1. Shotshell sabot slug cross section with major components identified

Figure 2. Rifle cartridge cross section

Figure 3. Revolver cartridge cross section

Figure 4. Boxer primer cross section

Figure 5. Berdan primer and case cross section

Figure 6. End view of Berdan-primed case

Figure 7. Shotshell primer cross section

Figure 8. Primer sensitivity test example

Figure 9. Close-up of Boxer primer showing primer crimp

Figure 10. Primer cup location at firing pin strike

Figure 11. Primer pocket pressure pushes primer cup aft, case forward

Figure 12. Primer cup bulges, case moves aft

Figure 13. A 25-06 case with blown primer

Figure 14. Extractor groove diameter for a normal case and blown primer case

Figure 15. Sketches of fired .50-caliber case and primer component

Figure 16. Cartridge case’s stress-strain behavior

Figure 17. Contact time vs. case location and case type

Figure 18. Cartridge case strain components

Figure 19. CASAS cartridge case model

Figure 20. CASAS case ring forces

Figure 21. Finite element model of case, chamber, and breech bolt

Figure 22. Stress contours in finite element model of case, chamber, and breech bolt

Figure 23. Externally scribed case

Figure 24. Scribed case-measurement equipment

Figure 25. Case max percent ultimate strain vs. peak pressure and case body taper

Figure 26. Peak bolt load vs. peak pressure and case body taper

Figure 27. Residual case base-bolt face load vs. Pmax and case body taper

Figure 28. Peak initial extract load vs. Pmax and case body taper

Figure 29. Remington Model 742 bolt

Figure 30. Zones vs. axial locations for cases A and B

Figure 31: Brass case A’s stress vs. strain properties

Figure 32. Brass case B’s stress vs. strain properties

Figure 33. Typical brass cartridge case hardness gradient

Figure 34. Photo of 20mm brass case exposed to marine atmosphere

Figure 35. Verdigris on 30-06 cartridge cases

Figure 36. Stress corrosion cracking of 20mm brass case

Figure 37. A 20-gauge case with cartridge corrosion rim split

Figure 38. 17 HMR cases split as a result of stress corrosion cracking

Figure 39. Small caliber steel cartridge case

Figure 40. Steel case’s stress vs. strain properties

Figure 41. Photo of cadmium-plated steel cartridge case exposed to the elements

Figure 42. Aluminum pistol case with Berdan priming system

Figure 43. Aluminum case burn through

Figure 44. Aluminum case’s stress vs. strain properties

Figure 45. Photo of aluminum cartridge case exposed to the elements

Figure 46. Commercial plastic case development

Figure 47. Plastic case’s stress vs. strain properties

Figure 48. A 27×145mm cartridge case

Figure 49. Slug and shotshell payload cross sections

Figure 50. Photo of unburned and partially burned artillery propellant grain

Figure 51. Effect of varying peak pressure on percent ultimate strain vs. location

Figure 52. A .50-caliber case with material samples cut from the body

Figure 53. Cartridge case dog bone tensile specimen

Figure 54. Tensile test equipment and computer data acquisition system

Figure 55. Dog bone tensile sample tested to failure

Figure 56. Case percent ultimate strain vs. location and case type

Figure 57. Shoulder datum-controlled headspace

Figure 58. Flange-controlled headspace

Figure 59. Belt-controlled headspace

Figure 60. Photo of bolt with spring-loaded eject plunger

Figure 61. Effect of variable axial gap on case percent ultimate strain

Figure 62. Effect of variable radial gap on case percent ultimate strain

Figure 63. French MAS and Springfield 1903 rifle breech bolts

Figure 64. Percent ultimate strain vs. lock stiffness variation and axial location

Figure 65. Strain-gauged M16 barrel and bolt with cross pin

Figure 66. Effect of coefficient of friction scalar on percent ultimate strain

Figure 67. Peak bolt load vs. peak pressure and coefficient of friction scalar

Figure 68. 5.56mm (M855) brass case’s temperature vs. time

Figure 69. Ruger No. 1 breech block

Figure 70. Residual bolt load just prior to unlock

Figure 71. Extraction load

Figure 72. Residual case—chamber friction after unlock vs. location

Figure 73. Cases fired in standard and fluted chambers

Figure 74. .50-caliber muzzleloader, peak pressure—velocity for black powder and BP substitutes

Figure 75. Comparison of in-line muzzleloader breech faces

Figure 76. Propellant geometry options for small caliber systems

Figure 77. Effect of propellant geometry on progressivity

Figure 78. Schematic and cross section photo of Ball Powder grain with deterrent

Figure 79. Group size vs. muzzle velocity variation and load type

Figure 80. Group size vs. muzzle velocity variation for 4 calibers

Figure 81. Photos of 20mm sabot segment exteriors

Figure 82. Muzzle velocity nomogram

Figure 83. Single-perf propellant web definition

Figure 84. Mean pressure and standard deviations vs. travel, .30-caliber barrel

Figure 85. 45ACP peak pressure vs. in-bore travel

Figure 86. 45 ACP mass fraction vs. in-bore travel

Figure 87. Surface/volume ratio vs. web size of small caliber powders

Figure 88. Peak engraving pressure vs. projectile diameter and construction

Figure 89. Pressure and velocity vs. charge weight for new and worn barrels

Figure 90. Sticker trajectory classes

Figure 91. In-bore wear of metallic rotating bands

Figure 92. In-bore wear of plastic rotating bands

Figure 93. 9×19mm average MV vs. charge weight and OCL

Figure 94. 9×19mm mean MV and MV variation vs. charge weight

Figure 95. Solid projectile cross section

Figure 96. Composite projectile cross section

Figure 97. CG offset vs. PA tilt for front-loaded bullet

Figure 98. Body-fixed, 6-DoF trajectory, predicted impact points

Figure 99. CG offset vs. PA tilt for aft-loaded bullet

Figure 100. Aft-loaded bullet flight path deviation vs. feature offset

Figure 101. Examples of 12-gauge saboted projectile cartridges

Figure 102. Centerfire rifle saboted projectile cartridges

Figure 103. Projectile drift vs. range

Figure 104. Drag coefficient vs. Mach number for various Siacci reference projectiles

Figure 105. Projectile’s yawing motion (looking downrange)

Figure 106. Projectile angle of attack

Figure 107. Axial force

Figure 108. Normal force

Figure 109. Magnus force

Figure 110. Pitching moment computation

Figure 111. Pitching moment

Figure 112. Pitch damping moment

Figure 113. Roll decay moment coefficient

Figure 114. Magnus moment coefficient

Figure 115. Projectile aerodynamic features

Figure 116. Ogive shape types

Figure 117. Fast-arm and slow-arm dynamic instabilities

Figure 118. Standard and polygonal land geometries

Figure 119. 7.62mm dispersion vs. barrel twist and rifling geometry

Figure 120. Projectile’s angle of attack and aero jump vs. applied impulse location

Figure 121. Generalized muzzle exit flow field

Figure 122. Mach disk location for various weapons

Figure 123. Details of asymmetrically applied load in various twist barrels

Figure 124. Total angle of attack vs. range for bullets fired in various barrel twists with 5N external applied load

Figure 125. Drop vs. range for various twist barrels and 5N external applied load

Figure 126. Drift vs. range for various twist barrels and 5N external applied load

Figure 127. Induced angular rate and cross velocity for standard and polygonal rifling

Figure 128. CG offset-induced cross vel. and angular rate vs. land pressure angle

Figure 129. APFSDS-T projectile in-bore and in flight

Figure 130. Zero yaw drag vs. Mach number

Figure 131. Pitching moment coefficient derivative vs. Mach number

Figure 132. Gyroscopic stability factor vs. slant range

Figure 133. Magnus moment coefficient derivatives vs. Mach number

Figure 134. Total angle of attack vs. range

Figure 135. .22 rimfire ammunition targets at 100 yards

Figure 136. SARP 5 projectile

Figure 137. Center of pressure vs. Mach number

Figure 138. A 100-yard dispersion of increased length bullet

Figure 139. Improved bullet group size vs. land diameter and range

Figure 140. Unfired and fired air gun pellets with rifling marks visible

Figure 141. Wireframe image of .22 air gun pellet with CG and normal force CP

Figure 142. .22-caliber air gun pellet angular rate variation vs. aero trim and drag variation for constant dispersion

Figure 143. Error budget pareto for .22-caliber air gun pellet

Figure 144. Illustration of in-bore clearance

Figure 145. Photo of recovered small caliber bullet with evidence of in-bore tilt

Figure 146. Projectile’s center-of-gravity offset in-bore

Figure 147. Bore deflections vs. forcing cone angle

Figure 148. Comparison of average engraving pressure and standard deviations for two forcing cone angles

Figure 149. Peak engraving pressure vs. forcing cone half angle

Figure 150. Stress contours at 60,000 psi for Barnes TSX and M80 ball

Figure 151. Comparison of wheel base and CP location for secant and tangent ogive bullets

Figure 152. External grooves

Figure 153. Barrel land width

Figure 154. MK149 discard contact

Figure 155. Whack in the back angle of attack and jump

Figure 156. Static vs. dynamic barrel motion and pointing

Figure 157. Bore scope images of copper deposition

Figure 158. Notional dispersion error budget

Figure 159. Balloting model of Barnes’ 30 Caliber Bullets and 300 Win Mag Test Fixture

Figure 160. Dispersion error budget for .30 caliber 168g Barnes’ Triple Shock in 300 Win Mag test fixture

Figure 161. Error budget for .30-caliber 168g copper jacket / lead core bullet in 300 Win Mag Test fixture

Figure 162. Graphical definition of dispersion and accuracy

Figure 163. Observed launch drag var. vs. Mach number, exit twist, and land geometry

Figure 164. Dispersion vs. radar reduced average first maximum yaw

Figure 165. Dispersion troubleshooting flowchart

Figure 166. Pitch damping force coefficient vs. distance from projectile nose at M=2.5

Figure 167. Gun-target geometry illustrating effect of scope cant WRT bore centerline

Figure 168. Use of smart phone level app to true gun to gravity

Figure 169. Vertical alignment of scope reticle with external vertical surface

Figure 170. Scope level verification after installation and tightening scope mount tops

Figure 171. Photo and schematic of rotary forge

Figure 172. Moving mandrel ECM machining setup

Figure 173. Moving mandrel ECM schematic

Figure 174. Constant twist gang broach

Figure 175. Single-point cutting tool and cutting head

Figure 176. Photos of rifling buttons

Figure 177. Flow-forming manufacturing sequence

Figure 178. Barrel groove and land width

Figure 179. Barrel rifling twist options

Figure 180. Torque vs. travel for new and worn constant twist and gain twist barrels

Figure 181. Body engraved medium caliber projectile

Figure 182. Schematic showing free-floated barrel and mid-barrel support

Figure 183. Lateral load caused by projectile spin and CG offset

Figure 184. Stock support stiffness vs. contact distance from receiver

Figure 185. Dispersion error budget vs. stock contact support location

Figure 186. Small caliber gun drill bit

Figure 187. Medium caliber gun drill bit

Figure 188. Worst case bore deviations vs. bore diameter

Figure 189. Measured bore centerline deviations

Figure 190. Cycles to failure vs. fracture toughness and initial crack depth

Figure 191. Split barrel with aftermarket exterior flutes

Figure 192. Methods of measuring dispersion

Figure 193. Comparison of various methods for determining equivalent CEP

Figure 194. Dispersion variability vs. true dispersion and sample size

Figure 195. Ratio of average extreme spread and mean radius to standard deviation vs. sample size

Figure 196. MPI variability vs. sample size

Figure 197. Recovered projectile with evidence of in-bore tilt

Figure 198. Case length measurement

Figure 199. Browne and Sharpe cartridge runout gauge

Figure 200. Impact point data entry in spreadsheet

Figure 201. Formula for computing average (mean) impact points

Figure 202. Formula for computing standard deviation (sigma) of impact points

Figure 203. Ammunition dispersion vs. bullet type/weight and cartridge loaded length

Figure 204. Average impact points vs. bullet type/weight and loaded cartridge length

Figure 205. Small caliber cartridge case drilled for chamber pressure measurement

Figure 206. PCB conformal transducer

Figure 207. Barrel modifications required for PCB conformal transducer

Figure 208. Material properties graphical representation

List of Tables

Table 1. Min and max primer cup dimensions and strip thickness in inches

Table 2. Case-chamber interaction phases

Table 3. List of case interaction assessment parameters and results

Table 4. M16 lock stiffness assessment

Table 5. Powder and primer interaction effect on dispersion and MV variation

Table 6. The 300 Win Mag bullets, MV variation, and group size

Table 7. The 300 Win Mag factory ammunition, MV variation, and group size

Table 8. MV sigma range and remedies

Table 10. The 9×19mm load details in Glock 17

Table 11. Projectile component materials and physical properties

Table 12. List of aerodynamic coefficients and influences

Table 13. Air gun dispersion error budget factors and error magnitudes

Table 14. Dispersion vs. barrel twist, land geometry, and mean yaw

Table 15. A 175g Sierra Match King horizontal deflection vs. range for 1 degree scope cant angle

Table 16. Matrix of barrel features and manufacturing capability

Table 17. List of standard deviations from the mean, percent of population, and odds against

Table 18. Chronograph measurement errors

Table 19. Measured chamber and case mouth pressure for brass cases

List of Equations

Equation 1. Burn rate equation

Equation 2. Base pressure gradient

Equation 3. Ballistic coefficient definition

Equation 4. Greenhill equation

Equation 5. Miller twist formula

Equation 6. Gyroscopic stability equation

Equation 7. Dynamic stability equation

Equation 8. Closed-form jump equation

Equation 9. Total drag equation

Equation 10. Critical crack depth

Equation 11. MPI variability

How to Use This Book

This book is intended for small caliber ammunition design and production engineers, as well as more advanced shooters and reloaders who want a more in-depth understanding of how guns and ammunition work and interact with each other. As the reader will come to understand, as he or she reads and digests the contents of this book, there is very little that is truly simple about small arms ammunition. As a shooter and reloader, I acquired nearly all the technical reference books I could to better understand the how and why of ammunition performance. Some of the books were useful; others, not so much. I wrote this book to ensure the technical knowledge acquired during my career was shared with like-minded shooters and reloaders to assist them when they troubleshoot the mechanical and dispersion performance of their firearms.

This book can be used as a reference document, allowing the reader to find a topic in the table of contents or index and access that topic directly. For the user to get the full flavor, it’s best to read this book starting at the beginning and proceeding through in linear fashion without skipping any topic. The War Stories can be skipped except for the Lessons portion, unless the reader desires an amusing anecdote about learning from the front row in the school of hard knocks. Lessons learned that are listed at the end of the War Stories should help shorten the learning/design process for most shooters and reloaders and even some engineers.

Lastly, I don’t claim to have all the knowledge there is regarding ammunition, much less on small caliber ammunition. I frequently use experiences learned from medium and large caliber into my discussions and examples; the reader should be aware that scale is a primary consideration in only a few areas of projectile performance, and those are identified, to the best of my understanding.

Acknowledgments

This book is dedicated to everyone at Arrow Tech Associates, without whom the accumulation of knowledge contained herein would have taken several lifetimes, instead of just part of one. They have been mentors and sounding boards, and without their kind patience and contributions, this book would not have been possible.

Mr. Tim Janzen, formerly of Barnes Bullets, pushed a copy of Vaughn’s Rifle Accuracy Facts in front of me in May 2008, which made me think long and hard about the structural response of rifle barrels in response to the firing event. Without this prodding and valuable feedback, we wouldn’t be where we are today in the understanding of the factors affecting dispersion (not accuracy, dammit) of small caliber rifles.

Many thanks to Scott Zaminer and Andrew Gray without whose assistance this book would not have been possible. Also, many thanks to Maire Zaminer, primary proofreader, punctuation czar, and English-usage guru. I’d be remiss if I also didn’t thank my hunting buddy, Keith Miller, nitpicker extraordinaire.

Many thanks also to Paul Kennedy of GDATP (General Electric Armament and Technical Products) and George Kontis of Knights Armament, who provided feedback and comments. Mr. Gary Schneider of Schneider Barrels also provided invaluable information.

Finally, deepest heartfelt thanks to my wife, Karen, who put up with eighteen months of late evenings sitting next to a distracted husband as he wrote this book.

Over the past forty years as a ballistics engineer, I’ve worked on nearly every caliber of ammunition ranging in caliber from 17 Mach IV to 8-inch howitzer and most everything in between. Generally speaking, medium and large caliber ammunition is a good place for a young engineer to cut his teeth because ammunition performance in these calibers is generally linear—if you put 2X in, you get 2Y out. The small caliber ammunition is decidedly nonlinear, which can be vexing if you don’t have the background to understand why, but supremely interesting because you never know where the next twist will end up. This book is also dedicated to small caliber engineers and wannabes who need better information to help troubleshoot their problems.

Ammunition Components

I suppose we should start at the beginning, putting first things first. It’s helpful if we first identify the major components of the various ammunition types we’ll be discussing. The ammo types we’ll be discussing in general terms in this book are

shotgun,

rifle, and

pistol.

Other types will be occasionally discussed throughout this book when the topic under discussion relates to the above-mentioned ammunition, and when needed, a 3D image of these ammunition types will be provided with the major components identified.

A cross section identifying the major components of a modern sabot slug shot shell specifically designed for big game hunting is shown in Figure 1, including a cross section of a barrel.

Figure 1. Shotshell sabot slug cross section with major components identified

The primer, located at the left-hand side of Figure 1, is the match that starts the powder (aka propellant) burning after being struck by the firing pin. As it burns, the powder provides the push needed to propel the projectile down the barrel. The over powder wad provides a positive seal for the propelling gases, preventing the gas from escaping forward between the bore and sabot (pronounced say’-bow, French for wooden clog). The cartridge case is shown in blue and provides the convenient container for the cartridge components and replaceable chamber seal for the firearm, preventing the gases from escaping aft toward the shooter. The sabot is a sleeve surrounding the projectile that centers the bullet in the barrel. Also shown here, but not specifically labeled, are little protrusions on the interior of the sabot that prevent the projectile from sliding forward and slipping out of the case during handling and feeding of the cartridge. The base of the sabot provides structural support for the base of the projectile as it is accelerated down the barrel. The roll crimp shown in Figure 1 retains the projectile/sabot assembly in the case prior to firing as the cartridge is being handled, loaded, and chambered. Once free of the barrel, the sabot peels away from the projectile because the spin imparted by the rifling opens the sabot petals, and the resulting aerodynamic drag separates the sabot from the projectile, which continues downrange to the target unimpeded. By the way, firing one of these in a smoothbore shotgun will be a very big disappointment at ranges where you’d expect the ol’ punkin’ ball to perform well.

Figure 2 shows the cross section of a rifle cartridge with the major components and features identified.

Figure 2. Rifle cartridge cross section

Figure 3 shows a cross section of a revolver cartridge with the major features and components identified.

Figure 3. Revolver cartridge cross section

The good news is the basic concept of the cartridge case is essentially the same regardless of the application. There’s always some sort of projectile payload, whether it’s a single projectile or a swarm of them, like in a shotgun. There’s also a primer to get the powder going, and there’s always a quantity of powder or propellant to provide the push to get the payload moving down the barrel. In modern production firearms, there’s always a cartridge case that acts as a container to hold the whole smash together and provides a seal to prevent gas leakage aft toward the shooter. What differs among the different types of ammunition is a matter of detail, and that typically depends on the application. Each of the above-identified components will be discussed in some detail in the next pages, some in more detail than others.

There is a new ammunition technology that bears mention—that of caseless ammunition. Firearms that use this ammunition are in development for the government. The weight payoff by elimination of the case is a dramatic reduction in which the soldier has to carry for a given number of shots, as the cartridge case comprises about 35% of the total ammunition weight burden with the brass cases used currently. Systems employing caseless ammunition use a component of the firearm as the high-pressure seal instead of a portion of the cartridge, which is risky mechanically because the reused seal must positively prevent gas leakage, regardless of the cleanliness of the chamber and surfaces the seals contact and the temperature of the surfaces. Reliably providing this high-pressure seal shot after shot under the demanding conditions in an automatic gun is a daunting task. Any grit or gun gas residue remaining on the seal surfaces as the gun cycles new cartridges into the chamber can allow a high-pressure gas leak. As shown further on in this book, high-pressure gas leaks are highly detrimental to gun function (and the shooter), regardless of the material that comprises the seal. The last successful caseless technology involves the use of black powder, and the projectile is inserted into the firearm through the muzzle. To put it mildly, muzzle velocity and rate of fire are far from exciting compared to modern firearms. Considering where conventional guns are today, this is a large step in reverse. It is my prediction that caseless ammunition systems will remain a laboratory curiosity until nanomaterial technology is developed to provide self-healing, high-strength materials.

Primers

Primers are the pyrotechnic devices that start the combustion process in the cartridge case. They are typically set off by the impact of a firing pin, but primers have been designed such that they are initiated by an electrical impulse (combination of voltage and current), laser light, or in the case of specialized aerospace self-destruct circuits, shock. Remington developed an EtroniX cartridge initiated by an electric circuit. The intent is to significantly reduce the lock time (the time between application of pressure on the trigger and ignition of propellant in the cartridge case). That system is largely defunct, so in this book, we will focus primarily in percussion-initiated primers.

Some shooters spend extra money on benchrest primers. According to sources at various primer manufacturing operations, when you buy benchrest primers, what you’re spending your money on is a slightly larger quantity of primer mix, on average, than the standard primers and a slightly tighter tolerance regarding mix weight variation. Dimensionally, there are no differences in the metallic components because they are made on the same stamping lines as nonbenchrest primers, using the same tooling. Depending on where and how you’re going to use the ammunition, you might be able to talk yourself into using either type of primer. For formal target shooting by a skilled marksman or for hunting, you might want to take advantage of the larger quantity of primer mix and reduced variation in primer mix quantity afforded by benchrest primers, albeit at an increased cost. For plinking or for use by novice shooters, the standard primers are likely quite adequate. This is largely a decision you’ll have to make for yourself, depending on your particular application and needs.

Types of Primers

There are two primer sizes for small caliber ammunition, large and small, and two primer flavors; one intended for high pressure applications and the other for low pressure (e.g., pistol) applications. The result is a minimum of four different physical primer types; this does not include the aforementioned standard versus benchrest primer mix options. Table 1 lists the minimum and maximum primer cup dimensions and brass strip thickness from which the cups are formed.

Table 1. Min and max primer cup dimensions and strip thickness in inches

The cup’s outside diameter (OD) is designed to be a slight interference fit with the primer pocket of the case so that it is held in place by friction between the two components. Further, this interference ensures that the initial contact between the two parts expands into an effective high-pressure seal, preventing leakage of gun gas around the primer cup. Since there must be gaps present between the bolt body and the firing pin for the firing pin to move relative to the bolt body and strike the primer, the primer-cup wall must be sufficiently thick and strong to bridge these gaps and structurally survive at the peak pressure developed in the case.

For centerfire rifles and pistols, there are two types of priming systems—Boxer and Berdan. Both primer types were invented in 1886, and it’s ironic that Berdan primers were invented in the US but are now most commonly found in Eastern Europe, while Boxer primers were invented in England but are most frequently used in North America.

Cartridges using the Boxer primer system have a cavity in the base of the case called a primer pocket, a cup to contain the ignition pressure, and an anvil in addition to the ignition mix. The case also has a centrally located flash hole through which the gaseous detonation by-products travel to ignite the propellant. This priming system is widely used in the Americas and Western Europe, regions with governments that trust (at least on some level) their citizens with firearms. The central flash hole in the base of the case makes it quite convenient to use a punch to remove the primer quickly from the primer pocket. This makes reloading ammunition relatively simple. The central flash hole in the pistol and rifle cartridges can clearly be seen in Figure 2 and Figure 3.

Figure 4 shows a cross section of the Boxer primer with the major components identified, along with a top view of the anvil with the cuts through which the gas vents from the primer into the propellant bed of the cartridge case.

Figure 4. Boxer primer cross section

The Berdan priming system uses a cup to contain the ignition pressure and the primer mix just as Boxer primers, but with this ignition system, the anvil is an integral part of the cartridge case, and (typically) two flash holes are drilled or punched on either side of the integral anvil. This priming approach precludes easy reloading as the lack of a central flash hole prevents primer decapping by a central pin. This priming system is preferred in countries where an armed populace is a menace to the unelected government because it provides a means for the government to make reloading ammunition exceptionally difficult. It’s also used in a free society for cartridge cases made from materials with less structural margin than brass, mostly because we’ve been overrun with lawyers whose self-appointed duty it is to protect morons from themselves, regardless of the cost. A cross section of the Berdan primer is shown in Figure 5.

Figure 5. Berdan primer and case cross section

Figure 6 shows a photo of the mouth end of an aluminum case with a Berdan primer, intended to prevent easy reloading of the case.

Figure 6. End view of Berdan-primed case

Various less-than-optimal schemes have been cooked up over the years to decap Berdan-primed cartridges. Some involve a sharp spike driven into the side of the primer cup; others involve filling the case with water and using hydraulic pressure to force the primer cup out. Either way, you have a lot of hard, perhaps messy, work ahead of you. I’ve never bothered with decapping Berdan-primed cases in the past and can’t think of a good reason why I might be convinced to do it in the future. In fact, I can’t even think of a good reason to put it on my bucket list.

Shotshell primers are a self-contained igniter that have a cup, stamped anvil made from a single sheet of brass, and a stop flange (aka battery cup) that axially locates the primer with respect to the shotshell cartridge. A cross section of the shotshell primer and a top view of the stamped anvil are shown in Figure 7.

Figure 7. Shotshell primer cross section

The primary driver for the design of shotshell primers is the simple fact that shotshell cases are typically made from molded plastic with a stamped, brass-plated-steel metallic head to ensure positive extraction. The plastic hull can have a lot of give (e.g., low stiffness), causing the primer to potentially misfire even when solidly struck by the firing pin. For this application, it’s the battery cup that provides sufficient resistance to the firing pin’s impact to allow the primer cup to crush the primer mix, starting the propellant ignition.

Primer Sensitivity Tests

Primer lots are tested for safety and sensitivity by a formalized test that characterizes the energy required for 100% no function and 100% function. The test involves dropping a steel ball of a defined diameter on a fixture holding a firing pin surrogate above a series of sensitized primers, and in this case, the primers are pressed into cases, just like they would be if we were loading or reloading the cartridges. Fifty primers are tested at a baseline no-fire drop elevation and subsequent batches of fifty primers are tested at 1 in. drop height increments until an all-fire threshold is achieved. By this method, the performance of